Enzymes which initiate DNA excision-repair are being purified from human cells and tissue. These enzymes will be reacted with DNA modified by different types of carcinogens. The reaction products will be characterized by analytic techniques. These experiments will enable us to determine which types of carcinogenic damage are susceptible to endonuclease and/or glycosylase attack. Such experiments will elucidate the normal repair capacity of human cells for carcinogenic damage and will also aid our understanding of the defects in the congenital diseases associated with apparent repair deficiency.